1. Field of the Invention
The present invention relates to an infrared radiation absorbing film having good transparency and suited for use, among others, as a packaging material for perishable foods and as a covering material for green house.
2. Prior Art
For packaging foods and for covering protected horticultural houses, there have been mainly used polyethylene films, ethylene-vinyl acetate copolymer films, polypropylene films, polyvinyl chloride films and the like. The technology of providing these films with an infrared radiation absorbing property has been widely employed for improving the ripening behavior, preservability, taste and flavor of foods, for instance, in the field of packaging perishable foods and for improving the nighttime heat-retaining capacity of green house coverings.
While some synthetic resins used for preparing films, such as polyvinyl chloride resins and polyethylene terephthalate resins, themselves have good infrared radiation absorbing ability, other resins such as polyethylene and polypropylene resins are low in infrared radiation absorbing ability. For providing synthetic resins with higher infrared absorbing ability, a number of methods are known which comprise incorporating a certain specific inorganic compound into a synthetic resin to thereby make use of the ionic lattice vibration of such inorganic compounds upon exposure to infrared radiation.
Thus, for instance, Japanese Examined Patent Publication (Kokoku) No. 13853/1972 discloses a film for agricultural use which is excellent in transparency and heat retention and which is prepared by incorporating 1 to 10% by weight of silicon oxide into a polyethylene or an ethylene-vinyl acetate copolymer. Japanese Examined Patent Publication No. 5825/1982 discloses a film for agricultural use which is similar to that mentioned above and which is composed of a polyethylene or an ethylene-vinyl acetate copolymer containing an anhydrous aluminosilicate salt in an amount of 1 to 20% by weight.
However, while such synthetic resin films containing each of these inorganic compounds indeed have infrared absorbing ability, there is a problem that their transparency to visible light is impaired.
To solve such a problem, attempts have been made for example, to produce films having good transparency by adjusting the refractive index of an inorganic compound having infrared absorbing ability to the refractive index of the synthetic resin (Japanese Examined Patent Publication No. 50251/1984, Japanese Examined Patent Publication No. 5183/1984, etc.) or to add silicon oxide having a specified particle size to thereby prevent transparency decrease due to the formation of the so-called hard spots or granular structure as otherwise resulting from aggregation of silicon oxide particles (Japanese Examined Patent Publication No. 13832/1988).
The object, which is to solve the above problem, can be achieved to some extent by adding a specific inorganic compound to synthetic resins to thereby provide them with infrared absorbing ability while controlling the refractive index of said inorganic compound or using a specific particle size grade of said inorganic compound to thereby obtain highly transparent films. However, the effects are not yet satisfactory. Particularly when silicon oxide, which has high infrared absorbing ability, is added to synthetic resins and when said silicon oxide is very finely divided and has an average particle size of 4 .mu.m or less, the fine particles tend to aggregate. When films are produced, the aggregates readily cause formation of hard spots of tens of micrometers to several hundred micrometers in size in the films, impairing the transparency and appearance of the films produced. Interestingly, it has already been found that when a silicon oxide species having a larger average particle size is used, the above-mentioned hard spots formation rather less frequently takes place.
However, silicon oxide species having a larger particle size, although less likely to form hard spots, are disadvantageous in that the scattering of visible light is increased at the film surface and within the film with the result that the film transparency is decreased.
Furthermore, most synthetic resin films containing an inorganic compound as an additive have a further problem; namely, when they are stretched even if only a little, the stretched portions tend to become white and opaque.
This whitening phenomenon on stretching may be regarded as an indication of inferior quality in some instances in the field of packaging films or films for agricultural use.